Heat-sensitive recording materials, which produce a recorded image by thermally reacting a colorless or pale-colored leuco dye with a developer, which acts as an electron donor for the leuco dye, to develop a color, are well known.
Such heat-sensitive recording materials can be used, for example, in the areas of cash register receipts for POS (point-of-sale) systems, and paper for tickets. With the expansion of POS systems, the environments and methods in which heat-sensitive recording materials are used are diversifying, and use in harsh conditions is also increasing. Further, in such applications, heat-sensitive recording materials are also used as receipts. Therefore, the recorded area must have good preservability against oils, plasticizers, office supplies, hand creams, and various chemicals, as well as high suitability for stamping.
Since the color-developing reaction in heat-sensitive recording materials comprising a heat-sensitive coloring layer mainly comprising a leuco dye and a developer and formed on a support is a reversible reaction, color-developed images are known to fade with time. This color-fading reaction is accelerated in a high-temperature, high-humidity environment, and further progresses rapidly when the heat-sensitive recording layer is brought into contact with oils, plasticizers, etc., and color may fade to such an extent that recorded images become illegible. Further, food labels and labels attached to, for example, test tubes in hospitals may have a high-concentration alcohol solution dropped on them. This leads to background coloration, and color fading in the printed part, which in the worst cases may result in the recorded images becoming illegible. To overcome these problems, for example, PTL (Patent Literature) 1 proposes adding an epoxy compound into a heat-sensitive recording layer to improve preservability of recorded images, but sufficient effects against oils, plasticizers, etc., are not obtained. With recent progress in the development of developers with high preservability, for example, PTL 2 proposes adding a urea-urethane compound into a heat-sensitive recording layer. As a result, the above problems are being solved, but there is a problem of low recording sensitivity. To improve the stability (resistance to thermal background fogging) of unprinted portions, using a developer with a high melting point is effective. However, this method also has a problem of low recording sensitivity, and its application to recent high-speed printers or battery-operated handheld device printers is difficult.
To solve this problem, PTL 3, PTL 4, and PTL 5 propose using as a developer a combination of N-p-toluenesulfonyl-N′-3-(p-toluensulfonyloxy)phenylurea and 4,4′-bis[(4-methyl-3-phenoxycarbonylaminophenyl)ureide]diphenylsulfone or a combination of at least one diphenylsulfone-crosslinked compound and at least one specific hydroxydiphenylsulfone derivative. However, the heat-sensitive recording material disclosed in PTL 3 has a problem of discoloration (background fogging) of the blank-paper part with time. To prevent background fogging, the heat-sensitive recording material disclosed in PTL 4 comprises an organic compound containing nitrogen as a fogging inhibitor. However, because organic compounds containing nitrogen have strong color-fading properties, preservability decreases. Further, the heat-sensitive recording material disclosed in PTL 5 has improved preservability against oils and plasticizers in the recorded part, but has insufficient stability (resistance to thermal background fogging) in the unprinted part.
PTL 6 proposes using urea-urethane compounds, such as 4,4′-bis[(4-methyl-3-phenoxycarbonylaminophenyl)ureido]diphenylsulfone, as developers. However, while the compound disclosed in PTL 6 has excellent image preservability, its sensitivity is insufficient for uses requiring high sensitivity. Further, the compound disclosed in PTL 5 significantly reduces whiteness of a coating liquid comprising the compound due to coloration over time (liquid fogging), and a heat-sensitive recording material produced by applying this coating liquid also has significant coloration in the background portion (background fogging). Furthermore, when the heat-sensitive recording material is stored in a high-humidity environment, significant coloration of the background portion (resistance to wet background fogging) of the heat-sensitive recording material also occurs. There is also a problem that when the compound is used with other developers, liquid fogging and resistance to wet background fogging become even more significant.
To improve the resistance to wet-background fogging of a heat-sensitive recording material comprising 4,4′-bis[(4-methyl-3-phenoxycarbonylaminophenyl)ureido]diphenylsulfone, PTL 7 proposes heat-treating a dispersion produced by co-dispersion of 4,4′-bis[(4-methyl-3-phenoxycarbonylaminophenyl)ureido]diphenylsulfone in the presence of a coloring inhibitor such as silicate, and using the dispersion. However, due to its low recording sensitivity, the heat-sensitive recording material disclosed in PTL 7 has insufficient preservability against oils and plasticizers in the recorded part when applied to a high-speed printer or a battery-operated handheld device printer.
Further, with expansion of areas in which heat-sensitive recording materials are applied, and increased diversification and higher performance of recording devices, the environments where recording media are used are becoming more demanding. Not only high recorded-image quality, sensitivity, and image preservability, but also satisfactory long-term blank-paper preservability, i.e., initial properties, are required of the recording medium without deterioration even after long-term storage as blank paper before recording.
High-quality paper has been generally used as a support for heat-sensitive recording materials. In acidic papermaking, paper is produced by internally adding a rosin-based sizing agent and a filler such as clay and talc. Aluminium sulfate is generally used as a Fixing agent of the rosin sizing agent. Due to the sulfate radicals (sulfate ions) remaining in the paper, the pH on paper is in an acidic range. Accordingly, a color-developing substance contained in the heat-sensitive recording paper reacts with acidic ions on the paper surface and is more prone to cause background fogging during a long-term storage. Therefore, in order to prevent background fogging or reduce papermaking costs, neutral paper comprising an alkaline filler, such as calcium carbonate, is sometimes used as a support for heat-sensitive recording materials.
However, when neutral paper is used as a support for heat-sensitive recording materials, color-developing ability decreases before recording, or colors fade after recording, thereby making recorded images blurred, unclear, or, in some cases, almost illegible, during the storage of heat-sensitive recording materials, for example, within less than one year. In particular, when color-developing ability decreases before recording, decreased printing density of the heat-sensitive recording material causes difficulty in reading printed images, and the essential function as a heat-sensitive recording material is lost. Although the reason for the decrease in color-developing ability is not clear, it is presumed that a developer forms a salt with an alkaline filler contained in neutral paper and changes morphologically, thus resulting in decreased performance of the developer.
To solve the above problem, PTL 8 proposes using, as a support, neutral paper comprising an alkyl ketene dimer as a synthetic sizing agent, the zeta potential of a 0.02% dispersion or solution of the synthetic sizing agent (on a solids basis) at a pH of 8.0 being +20 mV or less. PTL 9 proposes forming a heat-sensitive recording layer comprising an alkali salt of a diisobutylene-maleic anhydride copolymer on neutral paper comprising an alkyl ketene dimer as a sizing agent. However, satisfactory results have not necessarily been obtained.